Field of the Invention
The present invention relates to an apparatus for testing semiconductor devices in order to find defective semiconductor devices in which the distance between the valence band and the conduction band in a defective semiconductor device is lower when compared with that of a defect-free semiconductor device.
Semiconductor devices such as, in particular, volatile and non-volatile memory chips, are presently tested at the wafer level in order to find defective semiconductor devices, such as, for example, memory cell transistors having so-called soft defects. These soft defects may be features of the semiconductor devices such as excessively high leakage currents, excessively low threshold voltages, etc.
As an example of semiconductor devices, wafer-level memory chips have previously been tested, in which the memory chips are subjected to disturbance cycles at an elevated temperature. First the memory chips are written to and then the temperature of the memory chips is increased. Finally, the memory chips are read at the elevated temperature.
During this procedure, the temperature increase causes an increase in the leakage currents and, consequently, a decrease in the retention time of the memory chips in order to be able to separate out from the latter those memory chips which have proved to be defective because of this xe2x80x9caging treatmentxe2x80x9d.
The above procedure has a disadvantage in that relatively long waiting times have to be observed for changing the temperature of the semiconductor chips. These long waiting times cause long test times, which result in a costly overall test program. Moreover, it must be remembered that different semiconductor devices require different temperature ranges for testing, which in turn necessitates different equipment for the corresponding test apparatuses.
It is accordingly an object of the invention to provide an apparatus for testing semiconductor devices which overcomes the above-mentioned disadvantageous of the prior art apparatus and methods of this general type. In particular, it is an object of the invention to provide an apparatus for testing semiconductor devices in which it is not necessary to wait for temperature changes so that long waiting times can be avoided, and in which, moreover, there is no need to make complicated changes to the equipment when testing different semiconductor devices.
With the foregoing and other objects in view there is provided, in accordance with the invention an apparatus for testing semiconductor devices to find defective semiconductor devices in which a distance between a valence band and a conduction band has a lower value as compared with that of defect-free semiconductor devices. The apparatus includes a tunable light source for projecting light onto semiconductor devices. The tunable light source is constructed to adjust the light to a specific wavelength and to a specific intensity and to project the light for a predetermined time so that when the semiconductor devices are irradiated with the light, electrons in defective ones of the semiconductor devices can be transferred into the conduction band from the valence band. In the defective ones of the semiconductor devices, the distance between the valence band and the conduction band has a lower value as compared with that of defect-free ones of the semiconductor devices.
In other words, the object of the invention is achieved by providing a tunable light source, which can project light onto the semiconductor devices. This light in each case is of a specific wavelength and a specific intensity for a respectively predetermined time so that when the semiconductor devices are irradiated with the light, electrons can be transferred into the respective conduction band from the valence bands that are located an excessively small distance away from the conduction band.
Thus, instead of temperature changes for finding memory cells having an increased leakage current potential, the apparatus according to the invention uses, during their test, the action of radiation energy emitted by a tunable light source. The wavelength of the light radiated by the light source enables direct setting of the energy that an electron in the semiconductor device receives in order to pass from the valence band into the conduction band. This energy is too small in the case of defective semiconductor devices. Since this jumping of the electrons from the valence band into the conduction band takes place practically instantaneously, long waiting times which inevitably occur in the case of temperature changes are obviated. Furthermore, because of the use of the tunable light source, there is no need to change or exchange the equipment, since the energy radiated by the light source or the wavelength of the light emitted by the light source can readily be adapted to the distance between valence band and conduction band, which distance is too small in the case of xe2x80x9cpoorxe2x80x9d semiconductor devices.
If the energy E required for transferring an electron from the valence band into the conduction band, which is too small in the case of xe2x80x9cpoorxe2x80x9d semiconductor devices, is 2.5 eV, for example, then a value of 497 nm is produced for the wavelength xcex with the aid of the known equations E=h*f and c=xcex*f (h=Planck""s constant; f=frequency of the radiated light; c=speed of light).
What is essential to the apparatus according to the invention, then, is the generation of a disturbance quantity by irradiating the semiconductor devices with a light source whose color temperature can be regulated in a continuously variable manner. In other words, a light source is used in which the wavelength of the radiated light can be preset. As a result, it is possible to directly set the energy which acts on the semiconductor devices, with the result that electrons can readily be moved from the valence band of the semiconductor devices into, if appropriate, a conduction band that is an excessively small distance away.
In accordance with an added feature of the invention, the tunable light source is built into a wafer sampler which subjects the semiconductor devices to a test cycle. In this wafer sampler, the semiconductor devices, that is to say in particular a wafer, can be adjusted below the light source. Equally, however, it is also possible to adjust the light source above the wafer. All that is important here is that the light source can be adjusted exactly with regard to the semiconductor devices, that is to say with regard to the wafer in the present example.
In accordance with an additional feature of the invention, instead of a light source, it is also possible, if appropriate, to use optical waveguides which can guide light from a remote tunable light source onto the semiconductor devices in a targeted manner.
The light source itself may, for example, be drivable via a customary interface, with the result that it can perform the radiation of light onto the semiconductor devices in a targeted manner.
During a test, by way of example, memory cells in a wafer are written to in a conventional manner. The wafer sampler is then driven in order to position the wafer below the light source of the wafer sampler. Information regarding a desired wavelength is subsequently output to the light source from a test program. In this case, this desired wavelength corresponds to an excessively small distance between valence band and conduction band of the memory cells. The light source is then switched on and off, depending on the test program. The wafer sampler is subsequently driven in order that it moves into a test position in which the memory cells irradiated by the light source can be tested. During this testing, the defective memory cells are then determined, in which memory cells, because of an excessively small distance between the valence band and the conduction band, electrons were transferred into the conduction band as a result of the irradiation by the light source.
During the above test operation, it must be ensuredxe2x80x94depending on the semiconductor devices respectively testedxe2x80x94that a possibly required voltage supply or driving by signals, such as, for example, refresher signals in DRAMs (Dynamic Random Access Memories, is not interrupted.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an apparatus for testing semiconductor devices, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.